Aircraft configuration for micro and mini UAV

ABSTRACT

An aircraft arrangement for Mini or Micro UAV comprising a fore wing ( 14 ) and an aft wing ( 12 ) in tandem closed-coupled arrangement. The aft wing ( 12 ) has side panels ( 18 ) and control surfaces ( 19 ), and tapered planform with positive sweep, while the fore wing ( 14 ) has non-positive trailing edge sweep. The fore wing ( 14 ) and the aft wing ( 12 ) are disposed at different height, and the aircraft arrangement has no other wings or tail arrangements.

CROSS-REFERENCE

This is a National Phase Application filed under 35 U.S.C. 371 ofInternational Application No. PCT/IL2004/000668, filed Jul. 22, 2004,the entire content of which is hereby incorporated herein by referencein its entirety.

FIELD OF THE INVENTION

This invention relates to tandem aircraft arrangements, in particularfor application in Micro and Mini UAV (Unmanned/Unihabited AircraftVehicles)

BACKGROUND OF THE INVENTION

The advance of technology in the end of 20th century, especially inmicro-technologies, including micro-mechanical systems, communicationand control devices, made it feasible to design and manufacture unmannedaircraft capable of performing various military and civilian missions.In 2001, about 50 US companies, academic institutions, and governmentorganizations have been developing over 150 UAV designs. About 115 ofthese designs are flying, while some 26 models of UAV's are inexploitation or ready for production. Most of the UAV used today are infact small airplanes measuring 2 m and more wingspan and capable tooperate at tens and hundreds of miles range.

In the 1990s, new classes of UAV started to develop: Mini and Micro-UAV.Mini-UAV are vehicles of about 20 cm to 1.2 m size while Micro-UAV arelimited to 6 inches (15 cm) in either dimension, according to thedefinition of Defense Advanced Research Project Agency (DARPA) of theUSA. The aerodynamic design of such small vehicles is not tractable bywell-developed methods applied to conventional aircraft, mainly due tothe low Reynolds number of their flight (2*10⁴-3*10⁵), and therequirement of low flight speed, which is comparable with moderate windspeeds, such as 10-20 m/s.

An example of Mini-UAV is the “Sender” of the US Naval ResearchLaboratory. It has a classical rectangular wing, tractor propeller andV-like empennage. The wingspan is 1.2 m and the flight weight is 4.5 kg.The vehicle is launched from a catapult and has near 100-mile rangecapability.

Very intensive research and design effort takes place in the Micro-UAV(MAV) applications. From the late 1990s, American universities stageyearly competitions of MAV, while governmental and commerciallaboratories pursue projects of mission-worthy vehicles.

MicroSTAR MAV of Sanders AS&T group and Lockheed is an operableprototype with flying wing configuration. The air vehicle has delta wingwith 9-inch span, side panels and tractor propeller driven by anelectric motor. It is capable to carry miniature video and IR cameras,communication and control means in a 2-mile range.

Intelligent Automation, Inc has designed and fabricated a 15 cm, 90 gMAV designed to fly at 40 mph for about 20 min, carrying a video camera,two-way RF communication receiver, and servomotors. The MAV hasclassical wing arrangement with swept wings, horizontal stabilizers,vertical keel, and tractor propeller driven by an internal-combustionengine.

The above-mentioned competitions and academia projects have brought upother aerodynamic designs seeking greater lift at reduced flight speedacceptable stability and controllability:

-   -   Mississippi State University “SKYDOG” with triplane wing        configuration, two keels, horizontal stabilizer connecting the        keels, and tractor propeller;    -   University of Notre Dame MAV with flying wing of inverse        Zimmerman planform (squared-off), central vertical keel and        tractor propeller;    -   University of Florida MAV with biplane tailless configuration.        Two vertical fins connect the two wings at ¾ of the semi-span        from the center plane. The upper wing is displaced about        half-chord to the rear.

A number of projects make use of the known fact in the aerodynamicdesign that double-wing configurations, such as biplane and tandem,provide greater lift and reduced induced drag in comparison to a singlewing configuration.

The “Outrider” UAV of Alliant Techsystems has two essentially straighttandem wings with 3.30 m span, with the forewing above and the aft wingbelow the fuselage. Side panels to form a rigid frame connect thewingtips. The vehicle has a T-like empennage.

DE 43 32 867 suggests a similar configuration but closer to biplanebecause the two wings are also overlapping each other.

U.S. Des. Pat. No. 166,843 suggest a light aircraft tandem configurationwith two swept wings, the forewing being above the fuselage, and apushing propeller. The aircraft has two vertical keels with ruddersbehind the aft wing.

Small vehicle size must be convenient to operate in close proximity to apoint of interest without being detected, loitering for 0.5-1 hour, andthen returning. The aircraft must be able to fly in turbulent and highwinds speed, to provide reasonable rate of climb, tight turns and bestable enough to serve as an airborne video platform. The aircraft mustbe easy enough to operate, so that soldiers may deploy it in the fieldwith minimal training.

There is a clear conflict between constrains imposed on dimensions ofMicro and Mini UAV and requirements to fly at reduced airspeeds withacceptable flying qualities and maneuver capabilities. Typicalconfigurations that try to achieve maximum exploitation of the allowablearea (rectangular, delta, ogive wings, etc.), suffer from pooraerodynamic efficiency, low values of maximum lift and problematicstability and control characteristics. The flight of these types ofconfigurations relies massively on sophisticated flight control, whiletheir performance is penalized by excessive weight of the battery(electric propulsion case) that should be carried for overcoming itspoor aerodynamic efficiency.

Typical configurations of MUAV use overpowered engines for overcomingits poor aerodynamics efficiency and suffer from excessive weights ofbatteries, engine plant and airframe. In flight, this results inincreased air speed, which harms loitering performance, take off/landingcharacteristics and quality of video display.

SUMMARY OF THE INVENTION

Glossary

Biplane: Airplane with two overlapped wings spaced vertically.

Control surface: Aircraft component used for trim and for producingrequired orientation and maneuver of the aircraft.

Dihedral angle: Design angle between a wing surface and horizontalplane.

Directional stability: The aerodynamic capability of an aircraft toreturn to its original attitude after a small instantaneous disturbancein yaw or/and roll attitude.

Lift: The component of the aerodynamic force, which is perpendicular tothe freestream velocity vector

Lifting surface: Any wing-like component that provides lift.

Longitudinal stability: The aerodynamic capability of an aircraft toreturn to its original attitude after a small instantaneous disturbancein pitch attitude.

Side panel: A lifting surface joined perpendicularly or almostperpendicularly at the wing tip chord.

Stall: Aerodynamics phenomenon in which aircraft wing experiencesincreased air resistance and decreased lift, due to flow separation onthe wing at great angle of attack or/and low speed. Stall limits maximumattainable lift.

Stall speed: The minimal airspeed that may be attained in straight levelflight at given conditions of weight and height.

Sweep (angle) of wing line: Angle measured between a line on the wing(such as a leading or trailing edge) and a perpendicular to the rootchord. The sweep is positive if the tip end of the line is further aftthan its root end.

Sweep of wing (wing sweep): Sweep angle of the line connectinghalf-cords of the wing. A swept wing has positive wing sweep.

Tandem: Wing arrangement of two wings shifted forward and backward.

Taper: The ratio between wing's tip chord and root chord.

Twist (angle): The angle between the chord line of a specific wingsection and the chord line of the wing root section.

Wing: Aircraft component providing lifting force (lift) to support theaircraft in the air. A wing is defined by airfoil sections along thewing span.

Wing area: The plan surface of the wing, including a portion of the areaadjacent to the centerline and covered by the fuselage.

Wing span: Distance between wing tip's chords measured perpendicularlyto the centerline.

In accordance with the present invention, there is provided an aircraftarrangement for Mini or Micro UAV comprising a fore wing and an aft wingin tandem closed-coupled arrangement. The aft wing has swept-taperedplanform with side panels and control surfaces. The forewing and the aftwing are disposed at different height, and the aircraft arrangement hasno other wings or tail arrangements.

Preferably, the aft wing has straight leading edge with positive sweepangle while the fore wing has straight trailing edge with negative sweepangle. The wing tips may be rounded. A portion of the aft wing trailingedge may have negative or positive sweep angle.

The aft wing is mounted on a fuselage and the forewing is preferablymounted to the upper side of the fuselage on one or more pylons. Theforewing is preferably higher than the aft wing at least by an averageaft wing cord, and may partially overlap it.

If the tandem arrangement has overall width W and overall length Lincluding any control surfaces, preferably the sum of planform wingareas of the tandem arrangement is at least 70% of the product W×L,preferably close to 75%.

The aft wing may have aspect ratio between 2.5 and 4, preferably closeto 3.

The projected areas of the aft wing and the fore wing are preferably inratio between 2:1 and 1:1.

The aft-wing preferably has rudder control surfaces on its side panels.The forewing also may have side panels with rudder control surfaces.

The aircraft preferably has a tractor propeller mounted in front of thetadem wing arrangement.

The aft wing and/or the forewing may have non-zero dihedral angle,preferably such that the vertical distance between tip chords of thefore wing and the aft wing is greater than the vertical distance betweentheir respective root chords.

The fore and/or aft wing may have variable twist along their span.Preferably, the aft wing has positive angle of incidence. Preferably,the aft wing has airfoils with positive zero-lift pitching moment.

In one embodiment of the present invention, the fore wing, the aft wingand other elements of said UAV are disposed so as to providelongitudinal aerodynamic instability. In this case, the self-propelledUAV may have a pushing propeller and the aircraft arrangement may havenegative pitching moment at zero-lift.

The proposed tandem arrangement will be hereinafter called “X-plane”because of its characteristic planform best seen in FIG. 1C and becauseof it's front view best seen in FIG. 1D.

Advantages

The proposed aircraft arrangement for Mini and Micro UAV, in its stableconfiguration, provides simultaneous improvement of lift-carrying andmaneuver capabilities, extended speed range, enhanced aerodynamicefficiency, control power in longitudinal and directional stabilityplanes, aerodynamic passive self-recovery at stall in the pitch planethat guarantee a safe flight in gusty air.

The close-coupled wings provide maximum exploitation of the area(load-carrying capability) together with improved aerodynamicefficiency, enhanced control power and acceptable stability.

The side panels mounted on the aft wing provide an improved effectiveaspect ratio of the configuration that reduces the induced drag.

The pylon/s and wing dihedral produce the spacing between the wing'stips that helps to reduce induced drag and provide loading distributionthat is favorable for maximum lift.

Known MiniMicro UAVs have some difficulties to answer performancerequirements for realistic weight of components and payload. On thecontrary, the X-Plane is capable to comply with the followingoperational requirements:

-   -   Payload of 25% of take-off weight;    -   Minimum speed of 10 m/s;    -   Minimum flight time of 1 hr;    -   Rate of climb greater than 300 ft/min;    -   Turn radius less than 10 m;    -   Acceptable flying qualities.

The principal advantage of the X-Plane is its feature of packagesolution that provides a simultaneous answer to all significantaeromechanical aspects in development of Micro/Mini UAV, helping tobridge between conflicting requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIGS. 1A, 1B, 1C and 1D are schematic views of a basic UAV configurationin accordance with the present invention;

FIG. 2 is a diagram of wing twist distribution along the wingspan of theforewing;

FIG. 3 is a diagram of wing lift coefficient of wing components;

FIG. 4 is a diagram of spanload distribution at maximum lift of the foreand aft wing illustrating stall self-recovery property of the X-plane;

FIG. 5 is a diagram of the wing lift and overall lift coefficientsillustrating the beneficial effect of the side panels and the couplingof the two wings;

FIG. 6 is a diagram of the X-plane polar C_(L)/C_(D) illustratingbenefits of the induced drag reduction v/s increased wetted area in theX-plane configuration.

FIG. 7 is a diagram of the lift coefficient C_(L) v/s the pitch momentcoefficient C_(M) for the self-trimming X-plane arrangement.

FIG. 8 shows two preferable airfoils used in the X-plane arrangement.

FIG. 9 is an illustration of the increased elevon arm l in the X-planearrangement v/s the elevon arm in a single-wing arrangement.

FIGS. 10A, 10B and 10C are schematic views of an UAV configuration withadditional features, in accordance with different embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIGS. 1A, 1B, 1C and 1D, there is shown an aircraftarrangement “X-plane” of a self-propelled Mini- or Micro-UAV 10comprising a fore wing 12 and an aft wing 14 in tandem arrangement 15,mounted on a fuselage 16.

The aft wing 14 is mounted on the fuselage 16 as lower wing and theforewing 12 is mounted on a pylon 17 to the fuselage 16 as upper wing.

The aft wing 14 has swept-tapered planform with negative (downward)dihedral angle, with control surfaces (elevons) 20. The aft wing alsohas side panels 18 with rudder controls 19. The forewing 12 has alsotapered planform but with negative sweep, and dihedral with positive(upward) dihedral angle.

The X-plane 10 includes also a tractor propeller 24 with power plant 28,batteries 30, payload 32 and others components (not illustrated). Thewings and the other elements of the UAV are disposed so as to providelongitudinal stability.

The two wings 12 and 14 of the X-plane have moderate aspect ratio, aboutthree for the aft wing and four for the forewing. The two wings aredisposed in close-coupled arrangement, where the average gap between thetrailing edge of the forewing and the leading edge of the aft wing isless than the forewing root chord. The wings may partially overlap eachother, so as to provide maximum area and benefit from biplane effect.(Overlapping is accounted as negative gap)

With reference to FIG. 2, the wings of the X-plane have span-wise twist.The wing sections airfoils types used for the aft wing are illustratedin FIG. 8( a), and for the fore wing, in FIG. 8( b). Symmetrical NACAairfoils are used for the side panels 18 and the pylon 17.

The major performance features of the X-plane configuration are asfollows:

Maximum Lift and Stall Self-Recovery

With reference to FIG. 3, the maximum lift C_(Lmax) of the X-planetandem configuration 15 is achieved at angle of attack α_(max) prior theaft wing stall at α_(Astall).

The spanload distributions of the fore wing and aft wing at stall (atC_(Lmax)) are illustrated in FIG. 4. Stall starts at the center of theforewing span while the aft wing has a proper margin to stall.

This provides built-in mechanism of passive-self recovery at stall inthe pitch plane and ensures effective exploitation of the area of thetandem arrangement 15 for generating maximum lift. These features of theX-Plane are provided by a proper design of wing angles of incidence,twist and airfoils aerodynamics characteristics.

The slipstream of front propeller assembly contributes to maximum liftespecially at high values of thrust coefficient.

Aerodynamic Efficiency

It should be understood that the requirement for maximal liftcoefficient at large angles of attack is a trade-off with theaerodynamic efficiency at cruise flight.

For the X-Plane, the coupling of two wings of moderate aspect ratio andthe beneficial effect of the side planes produce an effect of increasedeffective aspect ratio, coming with reduced level of induced drag andimproved aerodynamic efficiency—as shown in FIGS. 5-6 (the normalizationof aerodynamic coefficients being based on aft wing area).

Thus, the X-plane configuration allows achieving combined span loadclose to the optimum elliptical distribution, with minimal induced dragat cruise/loitering flight. The employed twist and taper design valuesalso serve this purpose.

Fore-Aft Wing Interaction

In designing specific span loads of the two wings, the mutualupwash/downwash effects with varying angle of attack can be accountedfor, in order to comply simultaneously with requirements of maximum liftand minimum induced drag at cruise lift coefficients.

The two pairs of wings staggered both in vertical and in horizontaldirection provide beneficial reduction of induced drag, improving theaerodynamic efficiency of the configuration. The load distributions ofaft/fore wings provide an improved maximum lift constrained by therequirement of passive self-recovery at stall (nose-down moment due tostall of fore wing).

The X-shape planform provides for beneficial interaction of theclose-coupled central wing parts while wing tips are spaced far apart toavoid adverse interaction.

Increased Effective Aspect Ratio

The beneficial effect of the side panels 18, together with the dragreduction due to wings interaction, produces the effect of increasedeffective aspect ratio, coming with reduced induced drag and improvedaerodynamic quality C_(L)/C_(D), as shown in FIGS. 5 and 6. Thisimproves the climb and endurance performance of the tandem configuration15, overcoming the defect of increased wetted area v/s the single wingoption. Reduced level of induced drag and improved values of maximumlift help to realize optimum range and endurance performance atincreased values of lift coefficients, complying, at the same time, witha required speed safety margin.

Self-Trimming

The inventive tandem configuration 15 is a self-trimmed arrangement withconstrained pitching moment. Such arrangement does not requiredeflection of control surfaces with associated trim drag losses at thedesign point of cruise flight (FIG. 7). For this purpose, theconfiguration is designed with positive zero-lift pitching moment(C_(M0)) that may be achieved in two ways:

employing airfoils with positive zero lift pitching moment (see FIG. 8,a).

The effect of the positive moments is enhanced by the larger localchords of the tapered aft wing 14;

employing twist distribution.

The slipstream of front propeller assembly may contribute to positiveC_(M0) at high thrust coefficient.

-   Thus, the employed positive zero lift pitching moment and/or twist    distribution satisfies, as a “package solution”, simultaneously all    requirements:    -   maximum lift and stall-self recovery;    -   self-trimmed at design point;    -   reduced induced drag.

Elevon Efficiency

With reference to FIG. 9, the suggested tandem arrangement 15 of twolifting surfaces (fore and aft) results in more forward location of theneutral point relative to single wing configuration 35. Since, forstable aircraft configuration, the center of gravity CG should belocated prior to the neutral point NP, the suggested arrangementproduces a significant increase of elevon arm l and improved controlpower in the longitudinal plane. Apart from enhanced maneuvercapabilities, this allows design for extended range of center of gravitylocations, including flight with increased static margin at high liftcoefficients. This feature is especially relevant for development of lowReynolds numbers Micro/Mini UAVs, where allowable static margins(minimum and maximum) and efficiency of control surfaces are the centraldesign issues.

Directional Stability

The forward location of center of gravity increases the arm of sidepanels 18 mounted on wing tips of the aft wing 14 in the horizontalplane, producing significant increase of directional stability of theconfiguration v/s the single wing option (similar to the increase ofelevon control power). This helps to achieve a favorable ratio betweenrolling and directional stability, ensuring acceptable flying qualitiesof the inventive configuration.

The above basic embodiment of the inventive tandem configuration may bechanged in various ways without deviating from the scope of the presentinvention. For example, the aft and fore wing may have rounded orpolygonal leading and/or trailing edges. The aspect ratio of the wingsmay be within the range of 2.5 to 4 for the aft wing and 3 to 5 for thefront one.

The forewing may have various planforms with smaller area than the aftwing and should be disposed very close to the aft wing (no more than onechord gap between wings) and possibly overlapping it.

With reference to FIGS. 10A, 10B and 10C, there is shown an arrangement50 of the X-plane with variations of wing form and additional elementsas compared to the basic configuration of FIG. 1. The forewing 52 is atapered wing with side panels 54, rudder controls 56, and elevators 58.The aft wing 60 has modified planform with partially reversed trailingedge in the area of the elevons 20.

The forewing may be mounted on two pylons above the fuselage, in orderto provide higher structural rigidity.

The modification of the X-plane with the elements shown in FIGS. 10A, Band C, has the following additional performance features:

Direct Lift Control

Simultaneous deflection of elevons 20 and elevators 58 (or, the wholefore wing) produces the effect of direct lift control—generation ofincremental lift without changing the orientation of the aircraft.

Side Force Control

Simultaneous deflection of fore wing rudders 56 and aft wing rudders 19on the side panels 54 and 18 respectively, produces an effect of sideforce control without producing yawing moment. Additional actuators arerequired for this case.

The X-plane may be designed also as unstable configuration. The unstableconfiguration allows further increase of maximum lift andmaneuverability, with negative values of the overall zero-lift pitchingmoment C_(M0). For this purpose, the airfoils employed in the wingdesign may have modest negative values of C_(M0). The increased elevonarm l (see FIG. 9) is very advantageous for the unstable configuration.In this case, part of the elevon travel is used for trim, while the restis reserved for control and maneuver of the air vehicle. Theself-propelled UAV with unstable X-plane configuration may be equippedwith a pushing propeller.

The invention claimed is:
 1. A self-propelled Micro UAV configured foraerodynamic flight at flight speeds in the range between 10 m/s to 20m/s at Reynolds numbers in the range between about 20,000 and about300,000, and comprising a fore wing and an aft wing in tandemclose-coupled arrangement, wherein a trailing edge of said fore wing isspaced from a leading edge of said aft wing by a positive gap, whereinan average value for said gap is less than a root chord of said forewing, wherein said aft wing has side panels and control surfaces on atleast one of said aft wing and said side panels, and a tapered planformwith positive sweep, said fore wing has non-positive trailing edgesweep, the fore wing and aft wing being disposed at different heights,and said arrangement being free of additional wings or tail arrangement,said Reynolds numbers being based on a characteristic chord length ofone of said fore wing and said aft wing.
 2. The UAV of claim 1, whereinsaid fore wing has straight trailing edges with negative sweep angle. 3.The UAV of claim 2, wherein said fore wing has negative sweep.
 4. TheUAV of claim 1, further comprising a fuselage.
 5. The UAV of claim 4,wherein said fore wing is unconnected from said aft wing independentlyof said fuselage.
 6. The UAV of claim 4, wherein said fore wing ismounted on the upper side of said fuselage on at least one pylon.
 7. TheUAV of claim 6, wherein said fore wing is disposed higher than said aftwing at least by the length of an average aft wing chord.
 8. The UAV ofclaim 1, wherein said fore wing and said aft wing partially overlap eachother in plan view.
 9. The UAV of claim 1, wherein the fore wing, theaft wing and other elements of said UAV are disposed so as to providelongitudinal aerodynamic stability.
 10. The UAV of claim 9, wherein saidarrangement has positive pitching moment at zero lift.
 11. The UAV ofclaim 1, wherein at least one of said aft wing and said fore wing hasrounded tips.
 12. The UAV of claim 1, wherein at least a portion of thetrailing edge of said aft wing has negative or positive sweep angle. 13.The UAV of claim 1, wherein said aft wing has aspect ratio between 2.5and
 4. 14. The UAV of claim 1, wherein said fore wing has aspect ratiobetween 3 and
 5. 15. The UAV of claim 1, wherein planform areas of theaft wing and the fore wing are in ratio between 2:1 and 1:1.
 16. The UAVof claim 1, wherein said control surfaces comprise rudder controlsurfaces on said side panels.
 17. The UAV of claim 1, wherein said forewing has side panels.
 18. The UAV of claim 17, wherein said fore winghas rudder control surfaces on its side panels.
 19. The UAV of claim 1,wherein said fore wing has control surfaces.
 20. The UAV of claim 1,wherein said self-propelled UAV has a tractor propeller mounted in frontof said tandem arrangement.
 21. The UAV of claim 1, wherein at least oneof said fore wing and said aft wing has non-zero dihedral angle.
 22. TheUAV of claim 21, wherein the dihedral angles of the fore wing and of theaft wing are such that the vertical distance between wing tips of saidfore wing and said aft wing is greater than the vertical distancebetween their respective wing roots.
 23. The UAV of claim 1, whereinsaid aft wing has twist.
 24. The UAV of claim 1, wherein said fore winghas twist.
 25. The UAV of claim 1, wherein said aft wing has positiveangle of incidence.
 26. The UAV of claim 1, wherein said aft wing hasairfoil sections with positive zero lift pitching moment.
 27. The UAV ofclaim 1, wherein the fore wing, the aft wing and other elements of saidUAV are disposed so as to provide longitudinal aerodynamic instability.28. The UAV of claim 27, wherein said self-propelled UAV has a pushingpropeller mounted after said tandem arrangement.
 29. The UAV of claim 27having negative pitching moment at zero-lift.
 30. The UAV according toclaim 1, wherein said UAV is a micro-UAV and has at least one of amaximum longitudinal length and a maximum wingspan not greater thanabout 15 cm.
 31. The UAV according to claim 1, wherein said UAV is amini-UAV and has at least one of a maximum longitudinal length and amaximum wingspan between about 20 cm and about 1.2 m.
 32. The UAV ofclaim 1, wherein said characteristic chord is an average chord takenbetween a root and a tip of said main wing.
 33. The UAV of claim 1,wherein said main wing is said aft wing.
 34. A self-propelled Micro UAVconfigured for aerodynamic flight at flight speeds in the range betweenabout 10 m/s to about 20 m/s at Reynolds numbers in the range betweenabout 20,000 and about 300,000, and comprising a fore wing and an aftwing in tandem close-coupled arrangement, wherein a trailing edge ofsaid fore wing is spaced from a leading edge of said aft wing by apositive flap, wherein an average value for said flap is less than aroot chord of said fore wing, wherein said aft wing has side panels andcontrol surfaces on at least one of said aft wing and said side panels,and a tapered planform with positive sweep, said fore wing hasnon-positive trailing edge sweep, said fore wing and said aft wing beingdisposed at different heights, and said arrangement being free ofadditional wings or tail arrangement, wherein said Reynolds numbers arebased on a characteristic chord length of one of said fore wing and saidaft wing, wherein said tandem arrangement of said fore wing and said aftwing has an overall width W and an overall length L including anycontrol surfaces of said UAV, and the sum of both fore and aft wingplanform areas of said tandem arrangement is at least 70% of the productW×L.
 35. A self-propelled Micro UAV configured for aerodynamic flight atflight speeds in the range between 10 m/s to 20 m/s at Reynolds numbersin the range between about 20,000 and about 300,000 and comprising afore wing and an aft wing in tandem close-coupled arrangement, wherein atrailing edge of said fore wing is spaced from a leading edge of saidaft wing by a gap, wherein an average value for said gap is less than aroot chord of said fore wing, wherein said aft wing has first sidepanels and control surfaces on at least one of said aft wing and saidside panels, and a tapered planform with positive sweep, said fore winghas non-positive trailing edge sweep, said fore wing and said aft wingbeing disposed at different heights, and said arrangement being free ofadditional wings or tail arrangement, wherein said Reynolds numbers arebased on a characteristic chord length of one of said fore wing and saidaft wing, and wherein a planform area of the aft wing is not less than aplanform area of the fore wing.
 36. The UAV according to claim 35,wherein planform areas of the aft wing and the fore wing are in ratiobetween 2:1 and 1:1.
 37. The UAV according to claim 35, wherein saidtandem arrangement of said fore wing and said aft wing has an overallwidth W and an overall length L including any control surfaces of saidUAV, and the sum of both fore and aft wing planform areas of said tandemarrangement is at least 70% of the product W×L.
 38. The UAV according toclaim 35, wherein said UAV is a micro-UAV and has at least one of amaximum longitudinal length and a maximum wingspan not greater thanabout 15 cm.
 39. The UAV according to claim 35, wherein said UAV is amini-UAV and has at least one of a maximum longitudinal length and amaximum wingspan between about 20 cm and about 1.2 m.
 40. The UAV ofclaim 35, wherein said characteristic chord is an average chord takenbetween a root and a tip of said main wing.
 41. The UAV of claim 35,wherein said main wing is said aft wing.
 42. The UAV of claim 35,wherein the fore wing comprises second side panels, said first sidepanels being different from said second side panels.